This invention relates to an electrically conducting material. More specifically, it relates to a ceramic material suitable for preparing electrodes for utilization as current collectors in the channel of a magnetohydrodynamic (MHD) generator.
The environmental conditions within an operating MHD channel are very severe, and strenuous physical demands are placed on electrodes in this environment. The plasma, which is an ionized gas or an inert gas seeded with an ionizing agent such as potassium, may reach temperatures up to 3000 K, while the surface of the electrode may reach about 2000 K. However, since the electrodes are attached to open rectangular frames which are generally of copper, the electrode-frame temperatures can be no more than about 600-1000 K. Thus, the electrodes must be capable of withstanding a temperature differential between electrode-plasma interface and the electrode-frame interface of up to about 1400 K. The electrode must be able to withstand erosive forces from the plasma as it passes through the duct at near sonic velocities. The electrode must either be protected from oxidation or be prepared from oxidation-resistant materials since many plasmas, depending upon the particular fluid and its source, are oxidizing (Po.sub.2 .congruent.10.sup.-3 atm.) at operating temperatures. The electrode must also be able to withstand the highly corrosive effects of gaseous or molten potassium and molten coal slag when present in the fluid. The electrodes must be able to withstand the effects of electrochemical reactions which occur due to the passage of direct electric current through the anode and cathode in the presence of an electrolyte, i.e. the potassium seeded coal slag. Finally, the electrode must be constructed of materials which are electrically conductive at the normal operating temperature of the electrode and which can withstand the thermal shock of sudden temperatures changes due to generator malfunction without the electrode separating from the channel or without electrode disintegration. Thus, it is a problem to find materials from which electrodes can be made which can withstand the rigor of such an environment.
Electrodes based on lanthanum chromite (LaCrO.sub.3) have long been considered as promising MHD electrode materials for channels operating without coal slag. LaCrO.sub.3 based electrodes have successfuly operated in clean-fuel open-cycle MHD test channels for times around 100 hours. However, the material is subject to corrosion by coal slag and alkali metal seed material leading to a loss of CrO.sub.x. This loss gives rise to excess La.sub.2 O.sub.3 which is a very hygroscopic oxide. The combination of chromium oxide loss and attack by water on the La.sub.2 O.sub.3 and seed interaction results in changes in volume which may cause a catastrophic mechanical degradation of the electrode material.
The Third US-USSR Colloquium on Magnetohydrodynamic Electrical Power Generation, Moscow, USSR, Oct. 20-21, 1976, page 419, describes work in Russia in which yttrium chromite was doped with calcium and strontium for use as MHD electrodes. Subsequent work on these materials in this country has shown that, while these dopants did increase electrical conductivity of the yttrium chromite, they also increased the vaporization rate of the chromite, to the extent that the material is unsuitable and electrodes prepared of the material would become unusable in an MHD environment containing coal slag or an alkali metal seed within a short period of time.